Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold.
Compressor and turbine wheels have very complex shapes in order to change the direction and speed of flow of the air/exhaust gases and the pressure thereof. The wheels comprise thin-walled blade sections of around 1 mm thickness that are attached at an angle of between 45° and 90° to a large section hub. The air or gas flows along passages defined between the blades and the housing. For example, in a compressor wheel the blades are initially shaped to draw in the intake air in a generally axial direction and are then curved outwardly to redirect the air to flow in a radial direction whilst at the same time applying a centrifugal force and accelerating the air to a high velocity. The air must then be projected at high pressure by the blade tips into an outlet volute chamber at the radial periphery of the wheel. The form of the blades is fundamental to the aerodynamic performance of the turbocharger wheels and has to be accurately specified and repeated on each blade. In addition to the complex profile of the blades, the wheel has undercuts and other sudden changes in surface contours. The complexities of shape in the wheels ensures that all of the current manufacturing methods such as, for example, casting or machining from forgings, have their own unavoidable disadvantages.
The most common method for producing turbocharger wheels at the present time is casting. This is a relatively low cost process that can produce accurately dimensioned products. In the method, liquid metals, for example, Ni base superalloys for turbine wheels and Al—Si alloys for compressor wheels, are poured into a ceramic or plaster mould that has previously been produced by forming it over a master pattern such as wax, the wax being removed by a suitable solvent or by heating prior to the alloy being poured into the mould. Once the metal has cooled to room temperature the ceramic or plaster is broken away to reveal the wheel. The initial wax pattern is usually produced by injecting molten wax into a die.
Aluminium, being of low weight and relatively low cost, is a preferred material in the manufacture of both compressor and turbine wheels. In the former case it is used in the form of a matrix and in the latter it is used as an alloying element for turbine wheels. One disadvantage associated with aluminium is that it is prone to oxide defects both before and during casting even in a vacuum or inert gas environment. This kind of defect is not easily controllable and it reduces the durability of the component dramatically as it is generally where fatigue failure is initiated. The durability of such wheels is consequently difficult to predict and, as a result, turbochargers are less reliable. Major efforts have been made in recent years to reduce the oxide effects in casting aluminium and nickel base superalloy wheels but to little or no avail.
A further difficulty associated with casting of turbocharger wheels lies in the control of the microstructure of the material. The complex shape of the wheel means that it is almost impossible to ensure consistent control of the shrinkage, gas porosity and homogeneity of microstructure in terms of grain size, dendrite size and second phase particle size and so the consistency of component quality is reduced.
To address the problems associated with casting, a recent development has been to cast the material into a billet, extrude it into a bar, cut the bar into pieces, forge those pieces and then machine each forged piece into the shape of the wheel by a multi-axis machine. In this process any defects such as oxide inclusions and porosity are removed during the extrusion, forging and machining operations. Also, fine and homogenous grain structure and second phase particles can be obtained. The consistency in the durability of wheels made in accordance with this process is much improved in comparison to those produced by conventional casting. Although the process affords repeatable production of durable wheels it is, in view of the number of stages, labour intensive and much higher in cost compared to the casting method.
Whilst it is desirable to have a manufacturing process that can repeatedly produce high quality turbocharger wheels there is a need to ensure that the process is at reasonable cost.
It is well known that semi-solid forming of metals can be used to produce products of high strength and ductility without shrinkage problems. Semi-solid forming is a term used to describe the processing of a metal alloy that is between its liquidus and solidus temperatures where it comprises a slurry of solid phase metal particles suspended in the liquid phase molten metal. The dendritic solid particles are modified (e.g. by agitation) so that they approximate to spheroids. The most popular methods of processing: thixocasting and rheocasting of metals are known to produce components at low cost and of a quality comparable to components machined from solid metals. In thixocasting, the semi-solid thixotropic billet is produced by cooling the slurry whilst the dendritic microstructure is modified until it is solid and then reheating it to the semi-solid state, where the billet contains about 30-70% liquid phase, immediately before injection or casting into a mould. In rheocasting the alloy is fully melted, then cooled to a temperature between liquidus and solidus where solid particles are surrounded by liquid eutectics, the microstructure is modified and the component is formed by injection or casting the material in its semi-solid state into a mould. Rheocasting is attractive in that it offers the possibility of providing a semi-solid material on demand ready for injection into a mould in contrast to thixocasting where material is effectively provided in batches of solid billets for reheating before injection.
In both cases the semi-solid material can be transferred into a high-pressure injection or die-casting machine and injected into a die. After the injected material solidifies, the die is removed from the machine and is opened to expose the designed part. The advantage of thixocasting is that the desired homogeneous microstructure and elimination of casting defects is more controllable, but a disadvantage is that it is of higher cost than rheocasting.
The process of semi-solid forming has heretofore not been considered for the manufacture of complex shapes such as turbocharger wheels. All the current applications of semi-solid processing are for the production of relatively simple shapes where there are no large variations is cross-sectional area or complex profiles such as those described above. Examples of such manufacturing methods are described in U.S. Pat. Nos. 5,630,466, 6,214,478, US patent application no. 2003205351 and European patent no. 0980730.
The thixotropic behaviour of metal alloys at a semi-solid state and application of the thixotropic behaviour to shape metal products has been the subject of significant research. The production of thixoformable alloys and producing simple manufacturing components using thixocasting and rheocasting are described in many patents such as, for example, U.S. Pat. No. 3,948,650, French patent 2141979, U.S. Pat. No. 5,630,466, SK10002001, U.S. Pat. No. 6,214,478 (which specifically describes the production of relatively simple thin-walled body parts for vehicles), U.S. Pat. No. 5,879,478, WO0053914, and EP0980730).
Most early research concentrated on aluminium-silicon alloys as the alloys have a relatively clear boundary of solidification sequence between aluminium particles and silicon eutectics. For instance, the most popular thixoformable aluminium alloys A356 (6.5-7.5% Si, <1% of each other elements) and its modification alloy A357, (adding about 0.03% Sr and increasing Mg content to increase strength) were widely applied to manufacturing automotive components. The most popular components can be summarised as (see R. DasGupta: Industrial Applications—The Present Status and Challenges We Face in the Proceedings of the 8th International Conference on Semi-Solid Processing of Alloys and Composites, Limassol, Cyprus, 21-23 Sep. 2004):                (1) Fuel rail manufactured by Thixocasting of alloy A357;        (2) Automatic transmission gear shift lever manufactured by Thixocasting of alloy A357;        (3) Engine mount manufactured by Rheocasting of alloy A357;        (4) Different types of engine bracket manufactured by Rheocasting of alloy A357;        (5) Upper control arm manufactured by Rheocasting of alloy A356;        (6) Suspension manufactured by Rheocasting of alloy A357; and        (7) Diesel engine pump body manufactured by Rheocasting of alloy A356        
The products made by this process have been given significant quality improvement over castings and cost benefit over machined from solid metals.
A general feature of all the products described above is the relatively simple shape: the ratio of thinnest part of the product to its thickest section is no greater than about 1:2, and a simple casting die can be used to manufacture the product. Moreover, the components mentioned above are designed for operation in relatively simple conditions and often benign environments, unlike turbocharger wheels, which work under very complex conditions caused by thermal cycles, speed cycles and gas pressure etc.
There are more than ten different methods to shaping thixotropic alloys. All use the same concept, i.e. obtaining semi-solid microstructure with spheroidal solid particles surrounded by liquid phase and then to form the semi-solid material.
It is an object of the present invention to obviate or mitigate the above and other disadvantages and to provide for a method and apparatus for manufacturing the complex shapes of compressor and turbine wheels for turbochargers using a semi-solid process.